OFDM communication apparatus

ABSTRACT

An orthogonal frequency division multiplexing (OFDM) transmission apparatus and method generate a transmission sequence including first data, second data, and known signals such that more known signals are allocated to the first data than to the second data within the transmission sequence. The transmission sequence is OFDM multiplexed into a sequence of OFDM signals and transmitted. A receiver of the OFDM signal sequence uses the known signals for estimating transmission paths of the communicated OFDM signals and improving the reception of the data to which they are allocated.

TECHNICAL FIELD

The present invention relates to a communication apparatus using theOFDM (Orthogonal Frequency Division Multiplexing) method (hereinafterreferred to as “OFDM communication apparatus”), and more particularly toan OFDM communication apparatus that performs coherent detection.

BACKGROUND ART

In conventional OFDM communication, a transmitting-side apparatustransmits a burst unit signal such as that shown in FIG. 1, for example,to a predetermined receiving-side apparatus, as adopted in IEEE802.11and the like, for example. As shown in FIG. 1, a burst unit signalincludes guard intervals (GI), a preamble for transmission pathestimation, and an information signal (data). In a burst unit signal,the transmission path estimation preamble undergoes IFFT (inverse fastFourier transform) processing, and the information signal undergoespredetermined modulation processing and IFFT processing.

The receiving-side apparatus detects the FFT (fast Fourier transform)processing start timing by calculating a correlation value between theIFFT-processed transmission path estimation preamble and thetransmission path estimation preamble in the received burst unit signal(received signal). The receiving-side apparatus then extracts thetransmission path estimation preamble and information signal from thereceived signal by performing FFT processing on the received signal inaccordance with the detected start timing. The receiving-side apparatusalso performs transmission path estimation using the extractedtransmission path estimation preamble, and performs information signaldemodulation using the result of transmission path estimation. By thismeans, the receiving-side apparatus can extract a demodulated signal.

However, there is the following problem in conventional OFDMcommunication as described above. Namely, in conventional OFDMcommunication as described above, the number of symbols in atransmission path estimation preamble in a burst unit signal is fixed(in FIG. 1, one symbol).

In general, when a transmission path estimation preamble with a largernumber of symbols is used on a fixed basis as a transmission pathestimation preamble, the error rate characteristics of a demodulatedsignal obtained by the receiving-side apparatus are good. However, as atransmission path estimation preamble is not an information signal,using a transmission path estimation preamble with a larger number ofsymbols is equivalent to increasing the proportion of a burst unitsignal occupied by superfluous information. That is to say, when atransmission path estimation preamble with a larger number of symbols isused, information signal transmission efficiency falls.

On the other hand, when a transmission path estimation preamble with asmaller number of symbols is used on a fixed basis as a transmissionpath estimation preamble, the proportion of a burst unit signal occupiedby superfluous information is reduced, and information signaltransmission efficiency is consequently improved. However, depending onconditions such as channel quality, there is a high possibility ofdeterioration of the error rate characteristics of a demodulated signalobtained by the receiving-side apparatus.

Thus, in above-described conventional OFDM communications, there is aproblem in that it is difficult to achieve both an improvement indemodulated signal error rate characteristics and an improvement ininformation signal transmission efficiency.

DISCLOSURE OF INVENTION

It is an object of the present invention to implement an OFDMcommunication apparatus that achieves both an improvement in demodulatedsignal error rate characteristics and an improvement in informationsignal transmission efficiency.

This object is achieved by determining the number of known signals fortransmission path estimation (transmission path estimation preambles) tobe inserted in a transmit signal (burst unit signal) in accordance withchannel quality with respect to the communicating party. Morespecifically, this object is achieved by estimating demodulated signaldeterioration factors at the communicating party using channel qualitywith respect to the communicating party, and also determining the numberof known signals for transmission path estimation to be inserted in atransmit signal based on the estimated deterioration factors.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing the format of a burst unit signalused in a conventional OFDM communication apparatus;

FIG. 2 is a schematic diagram showing the relationship between channelquality and demodulated signal error rate for different numbers oftransmission path estimation preambles;

FIG. 3 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 1 of the presentinvention;

FIG. 4 is a block diagram showing the configuration of thesynchronization section in an OFDM communication apparatus according toEmbodiment 1 of the present invention;

FIG. 5 is a schematic diagram illustrating a burst unit signal formatused by an OFDM communication apparatus according to Embodiment 1 of thepresent invention (first example);

FIG. 6 is a schematic diagram illustrating a burst unit signal formatused by an OFDM communication apparatus according to Embodiment 1 of thepresent invention (second example);

FIG. 7 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 2 of the presentinvention;

FIG. 8 is a schematic diagram illustrating reception levels of signalstransmitted by each subcarrier in an OFDM communication apparatusaccording to Embodiment 2 of the present invention (first example);

FIG. 9 is a schematic diagram illustrating reception levels of signalstransmitted by each subcarrier in an OFDM communication apparatusaccording to Embodiment 2 of the present invention (second example);

FIG. 10 is a block diagram showing the configuration of the delayvariance detection section in an OFDM communication apparatus accordingto Embodiment 2 of the present invention;

FIG. 11 is a block diagram showing the configuration of the delayvariance detection section in an OFDM communication apparatus accordingto Embodiment 3 of the present invention;

FIG. 12 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 4 of the presentinvention;

FIG. 13 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 5 of the presentinvention;

FIG. 14 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 6 of the presentinvention;

FIG. 15 is a schematic diagram illustrating correlation valuescalculated by the synchronization section of an OFDM communicationapparatus according to Embodiment 6 of the present invention (firstexample);

FIG. 16 is a schematic diagram illustrating correlation valuescalculated by the synchronization section of an OFDM communicationapparatus according to Embodiment 6 of the present invention (secondexample);

FIG. 17 is a block diagram showing the configuration of the correlationpeak number detection section in an OFDM communication apparatusaccording to Embodiment 6of the present invention;

FIG. 18 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 7 of the presentinvention;

FIG. 19 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 8 of the presentinvention;

FIG. 20 is a schematic diagram illustrating a burst unit signal formatin an OFDM communication apparatus according to Embodiment 9 of thepresent invention;

FIG. 21 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 9 of the presentinvention;

FIG. 22 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 10 of the presentinvention;

FIG. 23 is a block diagram showing a sample configuration of acorrelator used in an OFDM communication apparatus;

FIG. 24 is a block diagram showing the configuration of the correlatorused in an OFDM communication apparatus according to Embodiment 11 ofthe present invention;

FIG. 25 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 12 of the presentinvention;

FIG. 26 is a schematic diagram illustrating reception levels of signalstransmitted by each subcarrier in an OFDM communication apparatusaccording to Embodiment 12 of the present invention (first example);

FIG. 27 is a schematic diagram illustrating reception levels of signalstransmitted by each subcarrier in an OFDM communication apparatusaccording to Embodiment 12 of the present invention (second example);and

FIG. 28 is a block diagram showing the configuration of the transmissionpath compensation section in an OFDM communication apparatus accordingto Embodiment 12 of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference now to the accompanying drawings, embodiments of thepresent invention will be explained in detail below.

EMBODIMENT 1

In this embodiment, a case is described where a transmission pathestimation preamble (known signal for transmission path estimation)inserted in a burst unit signal is varied adaptively in accordance withchannel quality.

First, an outline of an OFDM communication apparatus according to thisembodiment will be given with reference to FIG. 2. FIG. 2 is a schematicdiagram showing the relationship between channel quality and demodulatedsignal error rate for different numbers of transmission path estimationpreambles. FIG. 2 shows results of computer simulation concerning therelationship between demodulated signal error rate in a receiving-sideapparatus and channel quality (Eb/No: the ratio of power necessary fortransmitting a 1 bps signal to thermal noise power per Hz) for the casewhere the number of transmission path estimation preambles inserted in abus is 1 and the case where the number of transmission path estimationpreambles inserted in a bus is 2. The computer simulation conditions areas follows.

FFT sampling rate: 20 MHz; number of FFT samples: 64; guard intervallength: 800 ns; modulation method: 16QAM; FEC: convolutionalcoding/Viterbi decoding (coding rate: ¾; constraint length: 7) ; maximumDoppler frequency: 50 Hz; delay variance: 150 ns

As can be seen from FIG. 2, in the range up to an Eb/No value ofapproximately 29 dB, the Eb/No value for obtaining a predeterminedpacket error rate is approximately 1 dB better when the number oftransmission path estimation preambles is 2 than when the number oftransmission path estimation preambles is 1. That is to say, when Eb/Nois 29 dB or below, the demodulated signal error rate is better when thenumber of transmission path estimation preambles is 2 than when thenumber of transmission path estimation preambles is 1. However, in therange in which the Eb/No value is approximately 30 dB or above, there isalmost no difference in either case in the Eb/No value for obtaining apredetermined packet error rate. This phenomenon is due to the factorsdescribed below.

When the number of transmission path estimation preambles inserted in aburst unit signal is increased, thermal noise superimposed on eachtransmission path estimation preamble can be decreased in thereceiving-side apparatus by averaging the transmission path estimationpreambles in a received signal. Thus, with a demodulated signal obtainedusing a propagation path result estimated by means of a transmissionpath estimation preamble, deterioration due to thermal noise issuppressed, and consequently error rate characteristics are good. Thatis to say, increasing the number of transmission path estimationpreambles inserted in a burst unit signal is effective against errorrate deterioration due to thermal noise in a demodulated signal.

However, even if the number of transmission path estimation preamblesinserted in a burst unit signal is increased, it is not possible toimprove deterioration of error rate characteristics caused by adeterioration factor other than thermal noise (such as inter-codeinterference, synchronization errors, or frequency offset due tomultipath propagation, for example). That is to say, increasing thenumber of transmission path estimation preambles inserted in a burstunit signal is ineffective against error rate deterioration due to adeterioration factor other than thermal noise in a demodulated signal.

In FIG. 2, it is clear that thermal noise is the predominant factor inerror rate deterioration in the range in which Eb/No is approximately 20dB to approximately 30 dB, and a factor other than thermal noise is thepredominant factor in error rate deterioration in the range in whichEb/No is approximately 30 dB or above.

Therefore, when channel quality is within a certain approximate range(in FIG. 2, the range from approximately 20 dB to approximately 30 dB),it is possible to improve demodulated signal error rate characteristicsby increasing the number of transmission path estimation preamblesinserted in a burst unit signal, but when channel quality is withinanother certain approximate range (in FIG. 2, the range fromapproximately 30 dB upward), transmission efficiency falls whateverincrease is made in the number of transmission path estimation preamblesinserted in a burst unit signal, and demodulated signal error ratecharacteristics cannot be improved.

In light of the above, in this embodiment the number of transmissionpath estimation preambles inserted in a burst unit signal is variedadaptively in accordance with channel quality. Specifically, whenchannel quality is of a certain level (that is, when a factor other thanthermal noise is the predominant factor in deterioration of error ratecharacteristics, and consequently an improvement in the demodulatedsignal error rate cannot be expected even if the number of transmissionpath estimation preambles inserted in a burst unit signal is increased),the number of transmission path estimation preambles inserted in a burstunit signal is made as small as possible (in this embodiment, “1”).Conversely, when channel quality is not good (that is, when thermalnoise is the predominant factor in deterioration of error ratecharacteristics), the number of transmission path estimation preamblesinserted in a burst unit signal is increased.

By this means, it is possible to achieve both an improvement indemodulated signal error rate characteristics and an improvement ininformation signal transmission efficiency.

An OFDM communication apparatus according to this embodiment will now bedescribed with reference to FIG. 3. FIG. 3 is a block diagram showingthe configuration of an OFDM communication apparatus according toEmbodiment 1 of the present invention. An OFDM communication apparatusaccording to this embodiment comprises a receiving system and atransmitting system.

In the transmitting system, a modulation section 101 performs modulationprocessing on an information signal, and outputs the information signalthat has undergone modulation processing to a conversion section 103.Based on quality information from a detection section 115 in thereceiving system described later herein, a selection section 102 outputseither transmission path estimation preamble 1 or transmission pathestimation preamble 2 to the conversion section 103. As explained laterherein, the quality information from the detection section 115 isinformation indicating the quality of a demodulated signal in thereceiving system.

The conversion section 103 selects either an information signal that hasundergone modulation processing by the modulation section 101 or atransmission path estimation preamble from the selection section 102,and outputs it to an IFFT section 104. The IFFT section 104 performsIFFT processing on an information signal that has undergone modulationprocessing and a transmission path estimation preamble from theconversion section 103 and generates an OFDM signal, and outputs thegenerated OFDM signal to a guard interval (hereinafter referred to as“GI”) insertion section 105. The GI insertion section 105 inserts guardintervals in the generated OFDM signal and generates a transmit signal.The generated transmit signal is transmitted to the communicating partyvia an antenna 106.

Meanwhile, in the receiving system, a synchronization section 108outputs a signal received via an antenna 107 (a received signal) to anaveraging section 110 and selection section 111, and also calculates acorrelation value between the received signal and the IFFT-processedtransmission path estimation preamble, and detects the timing at whichthe calculated correlation value is at a maximum. To be specific, asshown in FIG. 4, the synchronization section 108 comprises a correlator301 that calculates a correlation value between the received signal andIFFT-processed transmission path estimation preamble, and a maximumvalue detection section (maximum detection section) 302 that detects thetiming at which the correlation value calculated by the correlator 301is at a maximum. The transmission path estimation preamble in thecorrelator 301 has the same signal pattern as the transmission pathestimation preamble to be inserted in a burst unit signal. Thissynchronization section 108 outputs the timing detected by the maximumvalue detection section 302 to a timing generation section 109.

Using the detected timing, the timing generation section 109 generates atiming signal that indicates the start timing of FFT processing in anFFT section 112, and outputs the generated timing signal to the FFTsection 112 that forms part of the demodulating means.

The averaging section 110 performs averaging on the received signal fromthe synchronization section 108 for a 2-symbol interval, and outputs theaveraged received signal to selection section 111. Based on informationindicating the number of transmission path estimation preamble symbolsinserted in a burst unit signal, stored in memory 114, selection section111 outputs either the received signal from the synchronization section108 or the averaged received signal from the averaging section 110 tothe FFT section 112.

The FFT section 112 extracts the signal transmitted by each subcarrierby performing FFT processing on the received signal from selectionsection 111. By this means, transmission path compensation is performedbased on the timing signal from the timing generation section 109. Ademodulation section 113 that forms the demodulating means together withthe FFT section 112 generates a demodulated signal by performingdemodulation processing on the signal extracted by the FFT section 112.This demodulation section 113 outputs the generated demodulated signalto the detection section 115 and also outputs information indicating thenumber of transmission path estimation preambles in the demodulatedsignal to the memory 114. This information indicating the number oftransmission path estimation preamble symbols is notified to thecommunicating party via a broadcast channel or the like. The memory 114stores this information indicating the number of transmission pathestimation preambles, and also outputs this information toabove-mentioned selection section 111.

The detection section 115 detects the quality of the demodulated signalfrom the demodulation section 113, and using the detection result,generates information indicating the quality of the demodulated signal(quality information). This quality information is output toabove-described selection section 102 in the receiving system.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described with additional reference to FIG.5 and FIG. 6. FIG. 5 is a schematic diagram illustrating a burst unitsignal format used by an OFDM communication apparatus according toEmbodiment 1 of the present invention (first example), and FIG. 6 is aschematic diagram illustrating a burst unit signal format used by anOFDM communication apparatus according to Embodiment 1 of the presentinvention (second example).

In the transmitting system, the transmission path estimation preamble tobe inserted in a burst unit signal is determined by the selectionsection 102 based on quality information from the detection section 115in the transmitting system. That is to say, when the quality of ademodulated signal in the receiving system is greater than or equal to athreshold value, based on recognition of the fact that a factor otherthan thermal noise is the predominant factor in deterioration of thedemodulated signal error rate, a transmission path estimation preamblewith a smaller number of symbols (the number of symbols here being “1”)is selected as the transmission path estimation preamble to be insertedin a burst unit signal. Conversely, when the quality of a demodulatedsignal in the receiving system is lower than a threshold value, based onrecognition of the fact that thermal noise is the predominant factor indeterioration of the demodulated signal error rate, a transmission pathestimation preamble with a larger number of symbols (the number ofsymbols here being “2”) is selected as the transmission path estimationpreamble to be inserted in a burst unit signal.

The threshold value used in selection section 102 can be set usingchannel quality at the critical point at which either thermal noise ispredominant or a factor other than thermal noise is predominant as afactor in deterioration of the demodulated signal error rate (forexample, Eb/No=30 dB in FIG. 2). In other words, the above-mentionedthreshold value can be set using channel quality at the critical pointat which demodulated signal error rate characteristics are eitherimproved or not improved by increasing the number of transmission pathestimation preambles inserted in a burst unit signal.

In this way, information indicating the number of symbols of thetransmission path estimation preamble selected by selection section 102is transmitted to the communicating party at the start of communicationvia a predetermined channel such as a broadcast channel or controlchannel, and is stored in the receiving system memory 114 at thecommunicating party. It is desirable for information indicating thenumber of symbols of the transmission path estimation preamble selectedby selection section 102 to be transmitted thereafter to thecommunicating party via the above-mentioned predetermined channel atpredetermined time intervals, and not only at the start ofcommunication.

The transmission path estimation preamble selected by selection section102 is output to the conversion section 103. An information signal isoutput to the conversion section 103 after being modulated using apredetermined modulation method (such as QPSK or 16QAM, for example).

The information signal that has undergone modulation processing by the101, or the transmission path estimation preamble from selection section102, is selected by the conversion section 103 and output to the IFFTsection 104. Specifically, when a transmission path estimation preamblewhose number of symbols is 2 is selected by selection section 102, the2-symbol transmission path estimation preamble and the informationsignal that has undergone modulation processing are output sequentiallyfrom the conversion section 103 to the IFFT section 104. Conversely,when a transmission path estimation preamble whose number of symbols is1 is selected by selection section 102, the 1-symbol transmission pathestimation preamble and the information signal that has undergonemodulation processing are output sequentially from the conversionsection 103 to the IFFT section 104.

In the IFFT section 104, IFFT processing is performed on thetransmission path estimation preamble and the information signal thathas undergone modulation processing from the conversion section 103.Specifically, the transmission path estimation preamble and theinformation signal that has undergone modulation processing are firstconverted from a single-sequence signal to a plurality of sequences ofsignals. Furthermore, by performing IFFT processing on the signal ofeach sequence, an OFDM signal is generated in which the signal of eachsequence is superimposed on a sequence-specific subcarrier.

The OFDM signal generated by the IFFT section 104 has guard intervalsinserted by the GI insertion section 105. By this means a transmitsignal is generated. Specifically, when a transmission path estimationpreamble whose number of symbols is 2 is selected by selection section102, the kind of burst unit transmit signal shown in FIG. 5 isgenerated. That is to say, a burst unit transmit signal is generatedthat contains a guard interval, a 1-symbol transmission path estimationpreamble, a guard interval, a 1-symbol transmission path estimationpreamble, a guard interval, and an information signal (data) comprisinga predetermined number of symbols. Each transmission path estimationpreamble in the burst unit signal shown in FIG. 5 has the same signalpattern.

Conversely, when a transmission path estimation preamble whose number ofsymbols is 1 is selected by selection section 102, the kind of burstunit transmit signal shown in FIG. 6 is generated. That is to say, aburst unit transmit signal is generated that contains a guard interval,a 1-symbol transmission path estimation preamble, a guard interval, andan information signal (data) comprising a predetermined number ofsymbols. It goes without saying that the transmission path estimationpreambles and information signal (data) in FIG. 5 and FIG. 6 have beensubjected to IFFT processing.

A burst unit transmit signal generated in this way undergoespredetermined transmission processing, and is then transmitted to thecommunicating party via antenna 106.

Meanwhile, in the receiving system, a signal transmitted by acommunicating party is sent to the synchronization section 108 viaantenna 107. The above-mentioned communicating party has the same kindof configuration as shown in FIG. 3. Therefore, a signal transmitted bythe above-mentioned communicating party undergoes the processingdescribed with reference to the transmitting system in FIG. 3.Furthermore, information indicating the number of symbols of thetransmission path estimation preamble selected by selection section 102at the communicating party is transmitted to this OFDM communicationapparatus at the start of communication via a predetermined channel suchas a broadcast channel, and is stored in the receiving system memory 114in this OFDM communication apparatus.

A received signal from antenna 107 is output to the averaging section110 and selection section 111 via the synchronization section 108. Inthe synchronization section 108, the correlation between the receivedsignal and an IFFT-processed transmission path estimation preamble iscalculated by the correlator 301, and the timing at which the calculatedcorrelation value is at a maximum is detected by the maximum valuedetection section 302. The detected timing is output to the timinggeneration section 109.

Using the timing detected by the maximum value detection section 302,the timing generation section 109 generates a timing signal thatindicates the start timing of FFT processing in the FFT section 112. Thegenerated timing signal is output to the FFT section 112.

In the averaging section 110, averaging is performed on the receivedsignal from the synchronization section 108 for a 2-symbol interval. Theaveraged received signal is output to selection section 111. Inselection section 111, based on information indicating the number oftransmission path estimation preamble symbols stored in the memory 114,either the received signal from the synchronization section 108 or theaveraged received signal from the averaging section 110 is selected asthe signal to be output to the FFT section 112.

Specifically, in the period in which a transmission path estimationpreamble is received, when the number of transmission path estimationpreamble symbols is 2, the averaged received signal from the averagingsection 110 is selected as the signal to be output to the FFT section112. The averaged received signal from the averaging section 110 at thistime is equivalent to a signal in which a signal corresponding to a1-symbol transmission path estimation preamble and a signalcorresponding to another 1-symbol transmission path estimation preambleare averaged. In this averaged received signal, thermal noise is reducedby averaging. Conversely, when the number of transmission pathestimation preamble symbols is 1, the received signal from thesynchronization section 108 is selected as the signal to be output tothe FFT section 112.

On the other hand, in the period in which an information signal (data)is received, the received signal from the synchronization section 108(that is, the signal corresponding to an information signal in thereceived signal) is selected as the signal to be output to the FFTsection 112, irrespective of the number of transmission path estimationpreamble symbols. In this way the signal selected by selection section111 is output to the FFT section 112.

In the FFT section 112, FFT processing is performed on the receivedsignal from selection section 111 based on the timing signal generatedby the timing generation section 109. This is equivalent to performingtransmission path compensation for the received signal based on a knownsignal for transmission path estimation (transmission path estimationpreamble). By this means, the signal transmitted by each subcarrier isextracted. The signals transmitted by each subcarrier are output to thedemodulation section 113.

In the demodulation section 113, a demodulated signal is obtained byperforming demodulation processing on the signals transmitted by eachsubcarrier from the FFT section 112. Specifically, the signalstransmitted by each subcarrier are converted from a plurality ofsequences of signals to a single-sequence signal. Then transmission pathestimation is performed using a signal corresponding to a transmissionpath estimation preamble in the single-sequence received signal. Usingthe transmission path estimation result, a demodulated signal isobtained by performing transmission path compensation on the signalcorresponding to an information signal in the single-sequence receivedsignal.

When a 2-symbol transmission path estimation preamble is inserted in aburst unit signal by the communicating party, as described above asignal is output from selection section 111 to the FFT section 112 inwhich the signal corresponding to a transmission path estimationpreamble in the received signal is averaged for a 2-symbol interval. Bythis means, thermal noise is reduced in the signal corresponding to atransmission path estimation preamble in the received signal, outputfrom the FFT section 112 to the demodulation section 113. Therefore, theeffects of thermal noise are also reduced in the demodulated signalobtained by the demodulation section 113. In this embodiment, the signalcorresponding to a transmission path estimation preamble in a receivedsignal prior to FFT processing is averaged for a 2-symbol interval, buta similar effect is also obtained if the signal corresponding to atransmission path estimation preamble in a received signal after FFTprocessing is averaged for a 2-symbol interval.

The demodulated signal obtained by the demodulation section 113 isoutput to the detection section 115. For the start of communicationonly, information indicating the number of transmission path estimationpreamble symbols in the demodulated signal is output from thedemodulation section 113 to the memory 114. This information is storedin the memory 114.

In the detection section 115, the quality of the demodulated signal fromthe demodulation section 113 is detected, and information indicating thedetected quality (quality information) is generated. Eb/No, receptionlevel information (RSSI), or the like can be used as a qualityindicator. The generated quality information is output toabove-mentioned selection section 102 in the transmitting system.

As stated above, in an OFDM communication apparatus according to thisembodiment, it is necessary to notify a communicating party of thenumber of symbols of a transmission path estimation preamble selectedbased on channel quality via a broadcast channel, control channel, orthe like. In order to notify a communicating party of this number oftransmission path estimation preamble symbols, an information amount ofonly 1 bit is necessary for one user (one communicating party). However,when, for example, 16QAM is used as the modulation method and the numberof subcarriers is 48, 192 bits of information can be transmitted by onesymbol. Thus, the amount of information needed to notify a communicatingparty of the number of transmission path estimation preamble symbols canbe said to be sufficiently small to be ignored when compared with theamount of information required for communication as a whole.

In this embodiment, a case where two kinds of transmission pathestimation preamble, with a number of symbols of 1 and 2 respectively,are used as transmission path estimation preambles inserted in a burstunit signal has been described as an example, but the present inventioncan also be applied to a case where three or more kinds of transmissionpath estimation preamble, each with a different number of symbols, areused. In this case, the transmission path estimation preamble with thesmallest number of symbols can be used when a factor other than thermalnoise is the predominant factor in deterioration of error ratecharacteristics, and consequently an improvement in the demodulatedsignal error rate cannot be expected even if the number of transmissionpath estimation preambles is increased. Conversely, the transmissionpath estimation preamble with the largest number of symbols can be usedon condition that the demodulated signal error rate does not meet apredetermined quality (for example, 0.01) when thermal noise is thepredominant factor in deterioration of error rate characteristics. Bythis means, it is possible to achieve both an improvement in demodulatedsignal error rate characteristics and an improvement in informationsignal transmission efficiency.

In this embodiment, a case has been described where the threshold valueused when selecting a plurality of transmission path estimationpreambles is set using channel quality at the critical point at whichdemodulated signal error rate characteristics are either improved or notimproved by increasing the number of transmission path estimationpreambles inserted in a burst unit signal. The present invention is notlimited to this, and can also be applied to cases where the thresholdvalue used when selecting a plurality of transmission path estimationpreambles is set by means of a variety of methods. For example, it isalso possible to for the channel quality necessary to obtain apredetermined error rate (for example, 0.01) for a demodulated signal(in FIG. 2, Eb/No=22 dB) to be used as the above-mentioned thresholdvalue. In this case, a transmission path estimation preamble with asmaller number of symbols can be used when the demodulated signalquality is greater than or equal to the threshold value, and,conversely, a transmission path estimation preamble with a larger numberof symbols can be used when the demodulated signal quality is lower thanthe threshold value.

The effect of improving transmission efficiency in this embodiment willnow be briefly described. When, for example, the packet length is 54bytes and 16QAM-R=¾, the number of OFDM symbols necessary forinformation signal (data) transmission is 3, and therefore the number ofOFDM symbols necessary for transmission of one packet of information is4. In this embodiment, as stated above, when channel quality is of acertain level, the number of transmission path estimation preamblesymbols inserted in a burst unit signal is changed from 2 to 1. If thenumber of users is 100, and 50 of the total number of users use atransmission path estimation preamble whose number of symbols is 1 aschannel quality becomes good, the transmissible amount of informationincreases by 9600 bits (192×50). That is to say, the transmissibleamount of information increases by 16.5%. If all 100 users use atransmission path estimation preamble whose number of symbols is 1, thetransmissible amount of information will increase by 33%.

Thus, in this embodiment, the number of transmission path estimationpreambles inserted in a burst unit signal is varied adaptively inaccordance with channel quality. Specifically, when channel quality isof a certain level (that is, when a factor other than thermal noise isthe predominant factor in deterioration of error rate characteristics,and consequently an improvement in the demodulated signal error ratecannot be expected even if the number of transmission path estimationpreambles is increased), the number of transmission path estimationpreambles inserted in a burst unit signal is made smaller. Conversely,when channel quality is not good (that is, when thermal noise is thepredominant factor in deterioration of error rate characteristics), thenumber of transmission path estimation preambles inserted in a burstunit signal is increased. By this means, it is possible to achieve bothan improvement in demodulated signal error rate characteristics and animprovement in information signal transmission efficiency.

In this embodiment, a case has been described where the quality of ademodulated signal obtained in the OFDM communication apparatus of thestation in question (the station's own OFDM communicationapparatus)—that is, the reception quality of a signal transmitted by acommunicating party—is used as a channel quality indicator, but it goeswithout saying that it is also possible, with the present invention, forthe quality of a demodulated signal obtained from reception quality at acommunicating party—that is, the reception quality at a communicatingparty of a signal transmitted by the apparatus in question—to be used asa channel quality indicator (this applies not only to this embodiment,but also to the embodiments described hereinafter). In this case also,it is similarly possible to achieve both an improvement in demodulatedsignal error rate characteristics and an improvement in informationsignal transmission efficiency.

EMBODIMENT 2

In this embodiment, a case is described where, as an addition toEmbodiment 1, not only reception level information but also multipathdelay time (delay variance) is used as a channel quality indicator.

There is a possibility that demodulated signal error ratecharacteristics will vary not only in accordance with reception levelinformation but also in accordance with the multipath delay time (thatis, the difference in arrival times of the principal wave and a desiredwave). Thus, it may not be optimal for the number of transmission pathestimation preambles to be inserted in a burst unit signal to bedetermined by means of reception level information alone. That is tosay, in general, there is greater deterioration of demodulated signalerror rate characteristics when the multipath delay time is long, andconversely, there is less deterioration of demodulated signal error ratecharacteristics when the multipath delay time is short.

Thus, in this embodiment, when the multipath delay time is long, alarger threshold value than in Embodiment 1 is used as the thresholdvalue used when switching between a transmission path estimationpreamble whose number of symbols is 1 and a transmission path estimationpreamble whose number of symbols is 2. That is to say, taking FIG. 2 asan example, whereas the threshold value used in Embodiment 1 is Eb/No=30dB, the threshold value used in this embodiment is Eb/No. 30 dB (forexample, 32 dB). When the multipath delay time is short, a smallerthreshold value than in Embodiment 1 may be used when switching betweena transmission path estimation preamble whose number of symbols is 1 anda transmission path estimation preamble whose number of symbols is 2.

FIG. 7 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 2 of the presentinvention. Parts in FIG. 7 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

As shown in FIG. 7, an OFDM communication apparatus according to thisembodiment has a configuration comprising, in addition to theconfiguration of an OFDM communication apparatus according to Embodiment1, a delay variance detection section 601 that detects delay variance(multipath delay time) using signals transmitted by each subcarrier, asize comparison section 602 that compares a detected delay variance witha threshold value REF, and a computation section 603 that performscomputation based on a comparison result on quality information obtainedfrom a detection section 115, and outputs post-computation qualityinformation to a selection section 102.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described with additional reference to FIG.8 through FIG. 10, considering only points of difference fromEmbodiment 1. FIG. 8 is a schematic diagram illustrating receptionlevels of signals transmitted by each subcarrier in an OFDMcommunication apparatus according to Embodiment 2 of the presentinvention (first example), FIG. 9 is a schematic diagram illustratingreception levels of signals transmitted by each subcarrier in an OFDMcommunication apparatus according to Embodiment 2 of the presentinvention (second example), and FIG. 10 is a block diagram showing theconfiguration of the delay variance detection section 601 in an OFDMcommunication apparatus according to Embodiment 2 of the presentinvention.

In the delay variance detection section 601, delay variance is detectedusing signals transmitted by each subcarrier from an FFT section 112.Specifically, delay variance—that is, multipath delay time—is detectedusing the difference in reception level between signals transmitted byeach subcarrier. That is to say, when the multipath delay time is short,the difference in reception level between signals transmitted byadjacent subcarriers is small, as shown in FIG. 8, and conversely, whenthe multipath delay time is long, the difference in reception levelbetween signals transmitted by adjacent subcarriers is large, as shownin FIG. 9.

First, therefore, as shown in FIG. 10, the absolute values of signalstransmitted by each subcarrier are output sequentially to a subtractionsection 902, and are also output to the subtraction section 902 afterbeing delayed by a predetermined time by a delay section 901.

In the subtraction section 902, the difference in reception levelbetween signals (absolute values) transmitted by adjacent subcarriers iscalculated. Calculated reception level differences are processed by anabsolute value calculation section 903 to find their absolute values,and then averaged by an averaging section 904. By this means the delayvariance is detected. Information indicating the detected delay varianceis output to the size comparison section 602. In the size comparisonsection 602, the delay variance detected by the delay variance detectionsection 601 is compared with a threshold value. The result of thecomparison is output to the computation section 603.

In the computation section 603, computation is performed on qualityinformation from the detection section 115 based on the result of thecomparison by the size comparison section 602. Specifically, when thedelay variance is greater than or equal to the threshold value (that is,when the multipath delay time is long), a predetermined value issubtracted from the quality information. Performing subtraction on thequality information in this way is effectively equivalent to increasingthe threshold value in selection section 102 (that is, using a largerthreshold value than in Embodiment 1 as the threshold value used whenswitching between a transmission path estimation preamble whose numberof symbols is 1 and a transmission path estimation preamble whose numberof symbols is 2). Conversely, when the delay variance is less than thethreshold value (that is, when the multipath delay time is short), nocomputation is performed on the quality information. Quality informationthat has been subjected to computation by the computation section 603 isout put to selection section 102 in the transmitting system.

Thus, according to this embodiment, by using not only reception levelinformation but also multipath delay time (delay variance) as a channelquality indicator, it is possible to achieve both an improvement indemodulated signal error rate characteristics and an improvement ininformation signal transmission efficiency irrespective of multipathdelay time.

EMBODIMENT 3

In this embodiment, a case is described where error in the multipathdelay time calculated in Embodiment 2 is reduced.

In general, in a receiving system, error may occur in a reception leveldue to error in automatic gain control in the radio section, or thelike. In this case, error occurs in the delay variance detected by thedelay variance detection section 601 shown in FIG. 7. Thus, in thisembodiment, the calculated delay variance is divided by the average ofthe reception levels of the signals transmitted by each subcarrier.

The configuration of an OFDM communication apparatus according to thisembodiment is the same as that in Embodiment 2 except for the delayvariance detection section 601. Therefore, only the internalconfiguration of the delay variance detection section 601 will bedescribed, with reference to FIG. 11. FIG. 11 is a block diagram showingthe internal configuration of the delay variance detection section 601in an OFDM communication apparatus according to Embodiment 3 of thepresent invention. Parts in FIG. 11 identical to those in Embodiment 2(FIG. 10) are assigned the same codes as in FIG. 10 and their detailedexplanations are omitted.

In FIG. 11, absolute values are found (that is, reception levels arecalculated) for signals transmitted by each subcarrier in an absolutevalue calculation section 1001, and these are then averaged by anaveraging section 1002. By this means, the average value of receptionlevels of signals transmitted by each subcarrier is calculated. In adivision section 1003, the delay variance calculated by averagingsection 904 is divided by the reception level average value fromaveraging section 1002. The new delay variance obtained in this way isoutput to the size comparison section 602 in FIG. 7.

Thus, in this embodiment, the calculated delay variance is divided bythe received signal level (the average value of reception levels ofsignals transmitted by each subcarrier), and the value obtained by thisdivision is used as a new delay variance. By this means, it is possibleto calculate a highly accurate multipath delay time even when erroroccurs in reception levels. It is therefore possible to achieve both animprovement in demodulated signal error rate characteristics and animprovement in information signal transmission efficiency irrespectiveof error in reception levels.

EMBODIMENT 4

In this embodiment, a case is described where, in Embodiments 1 through3, the number of transmission path estimation preamble symbols to beinserted in a burst unit signal is selected in accordance with the totalnumber of communicating parties (users) actually performingcommunication.

In an actual environment, not all users are necessarily performingcommunication (or communication is not necessarily being performed withall users), and therefore there are time slots in which communication isnot performed in a unit frame. A “unit frame” here contains only thepredetermined number of burst unit signals shown in FIG. 5 or FIG. 6. Tobe specific, when, for example, there are many communicating partiesperforming communication, a unit frame contains a larger number of burstunit signals and fewer time slots that are not used for communication.Conversely, when there are few communicating parties performingcommunication, a unit frame contains a smaller number of burst unitsignals and more time slots that are not used for communication.

Therefore, when there are few communicating parties actually performingcommunication, overall transmission efficiency will fall if atransmission path estimation preamble with a large number of symbols isinserted in a burst unit signal for predetermined communicating parties.On the other hand, when there are many communicating parties actuallyperforming communication, overall transmission efficiency will scarcelyfall even if a transmission path estimation preamble with a large numberof symbols is inserted in a burst unit signal for predeterminedcommunicating parties.

Thus, in this embodiment, the number of transmission path estimationpreamble symbols to be inserted in a burst unit signal is selected inaccordance with the total number of communicating parties (users)actually performing communication. That is to say, when the total numberof communicating parties actually performing communication is large, asmaller threshold value than in Embodiment 1 is used as the thresholdvalue used when switching between a transmission path estimationpreamble whose number of symbols is 1 and a transmission path estimationpreamble whose number of symbols is 2. And when the total number ofcommunicating parties actually performing communication is small, alarger threshold value than in Embodiment 1 may be used as the thresholdvalue used when switching between a transmission path estimationpreamble whose number of symbols is 1 and a transmission path estimationpreamble whose number of symbols is 2.

Although this kind of selection can also be applied to any ofEmbodiments 1 through 3, in this embodiment the case where this kind ofselection is applied to Embodiment 1 will be taken as an example, andwill be described with reference to FIG. 12. FIG. 12 is a block diagramshowing the configuration of an OFDM communication apparatus accordingto Embodiment 4 of the present invention. Parts in FIG. 12 identical tothose in Embodiment 1 (FIG. 3) are assigned the same codes as in FIG. 3and their detailed explanations are omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration comprising, in addition to the configuration of an OFDMcommunication apparatus according to Embodiment 1, a reception levelcalculation section 1101 that calculates the reception level of areceived signal from an antenna 107, an averaging section 1102 thatperforms averaging of calculated reception levels and calculates thereception level for a unit frame, a size comparison section 1103 thatcompares a calculated unit frame reception level with a threshold value,and a computation section 1104 that performs computation based on acomparison result on quality information from a detection section 115,and outputs the quality information subjected to computation to aselection section 102.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 1. In the reception level calculation section1101, the reception level of a received signal from antenna 107 iscalculated for each frame. Calculated received signal reception levelsfor each frame are averaged by the averaging section 1102, and thereception level is calculated for a unit frame. The reception level fora unit frame is output to the size comparison section 1103. In the sizecomparison section 1103, the unit frame reception level is compared witha threshold value. The result of the comparison is output to thecomputation section 1104.

In the computation section 1104, computation is performed on qualityinformation from the detection section 115 based on the result of thecomparison by the size comparison section 1103. Specifically, when theunit frame reception level is greater than or equal to the thresholdvalue (that is, when the total number of communicating parties actuallyperforming communication is large), a predetermined value is added tothe quality information. Performing addition to the quality informationin this way is effectively equivalent to decreasing the threshold valuein selection section 102 (that is, using a smaller threshold value thanin Embodiment 1 as the threshold value used when switching between atransmission path estimation preamble whose number of symbols is 1 and atransmission path estimation preamble whose number of symbols is 2).Conversely, when the unit frame reception level is less than thethreshold value (that is, when the total number of communicating partiesactually performing communication is small), no computation is performedon the quality information. Quality information that has been subjectedto computation by the computation section 1104 is output to selectionsection 102 in the transmitting system.

Thus, according to this embodiment, the number of transmission pathestimation preamble symbols to be inserted in a burst unit signal isselected in accordance with the total number of communicating parties(users) actually performing communication. In this way, by increasingthe number of transmission path estimation preamble symbols inserted ina burst unit signal for predetermined communicating parties when thetotal number of communicating parties actually performing communicationis small, it is possible to reduce time slots not used for communicationin each frame, and also to improve demodulated signal error ratecharacteristics at the above-mentioned predetermined communicatingparties. Conversely, by decreasing the number of transmission pathestimation preamble symbols inserted in a burst unit signal forpredetermined communicating parties when the total number ofcommunicating parties actually performing communication is large, it ispossible to increase the total number of burst unit signals contained ineach frame, and so improve transmission efficiency. Therefore, accordingto this embodiment, it is possible to achieve both an improvement indemodulated signal error rate characteristics and an improvement ininformation signal transmission efficiency.

EMBODIMENT 5

In this embodiment, a case is described where the number of transmissionpath estimation preamble symbols inserted in a burst unit signal isvaried in accordance with the communication channel.

In OFDM communication, there are specific channels for whichcommunication quality better than that of an ordinary channel isrequired. Such a specific channel may be, for example, a control channel(a channel used to send information indicating a burst to be received,information indicating the modulation method applied to an informationsignal, or the like), or a retransmission channel (a channel used tosend information for requesting retransmission).

The number of such specific channels is small compared with the totalnumber of channels. Therefore, overall transmission efficiency scarcelyfalls even if the number of transmission path estimation preamblesymbols inserted in a burst unit signal in a specific channel isincreased (in this embodiment, to two symbols) on a fixed basis. By thismeans, it is possible to improve the demodulated signal error rate in aspecific channel for which better communication quality is required withalmost no effect on overall transmission efficiency.

Increasing the number of transmission path estimation preamble symbolsinserted in a burst unit signal for such a specific channel on a fixedbasis will be described with reference to FIG. 13, taking the case wherethis procedure is applied to Embodiment 4 as an example (it goes withoutsaying that this procedure can be applied to any of Embodiments 1through 3). FIG. 13 is a block diagram showing the configuration of anOFDM communication apparatus according to Embodiment 5 of the presentinvention. Parts in FIG. 13 identical to those in Embodiment 4 (FIG. 12)are assigned the same codes as in FIG. 12 and their detailedexplanations are omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein computation section 1104 in an OFDM communicationapparatus according to Embodiment 4 is replaced by a computation section1201 that performs computation on a demodulated signal from a detectionsection 115 based on a comparison result from a size comparison section1103 and information indicating the communication channel.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 4. In the computation section 1201,computation is performed on quality information from the detectionsection 115 based on the result of a comparison by the size comparisonsection 1103 and information indicating the communication channel.Specifically, as in Embodiment 4, when the unit frame reception level isgreater than or equal to a threshold value (that is, when the totalnumber of communicating parties actually performing communication islarge), a predetermined value is added to the quality information.

Moreover, only when it is recognized from information indicating thecommunication channel that the communication channel is a specificchannel, a fairly large predetermined value is subtracted from thequality information. Subtracting a fairly large value from the qualityinformation in this way is effectively equivalent to making a fairlylarge increase in the threshold value in a selection section 102. Bythis means the number of transmission path estimation preamble symbolsinserted in a burst unit signal can be made two symbols on a fixedbasis. Quality information that has been subjected to computation by thecomputation section 1201 in this way is output to selection section 102in the transmitting system.

With an MAC (Media Access Control) section, for example, the timing atwhich a signal transmitted via a specific channel is received by thisOFDM communication apparatus is known. Thus, it is possible to useinformation generated by an MAC section as information indicating thecommunication channel that is input to the computation section 1201.

Thus, according to this embodiment, the number of transmission pathestimation preamble symbols inserted in a burst unit signal for acommunicating party is varied in accordance with the communicationchannel used to transmit that burst unit signal. That is to say, when,for example, a communication channel is a specific channel, the numberof transmission path estimation preamble symbols inserted in a burstunit signal is increased on a fixed basis, and therefore it is possibleto improve demodulated signal error rate characteristics in a specificchannel for which better communication quality is required. Conversely,when a communication channel is an ordinary channel, the number oftransmission path estimation preamble symbols inserted in a burst unitsignal is varied as described in Embodiment 1 through Embodiment 4. Bythis means, it is possible to improve the demodulated signal error ratein a specific channel for which better communication quality is requiredwith almost no effect on overall transmission efficiency.

In the above-described embodiment a control channel and a retransmissionchannel have been quoted as examples of a specific channel for which alarge number of preambles are used, but a specific channel of thepresent invention is not limited to these, and may include a broadcastchannel or the like, for example, the key point being the broadinclusion of channels for which communication quality better than thatof an ordinary channel (such as a user channel, for example) isrequired, as described above.

EMBODIMENT 6

In this embodiment, a case is described where the number of transmissionpath estimation preamble symbols selected by a communicating party isnot recognized by means of notification from that communicating party,but is estimated by means of a correlation value calculated using asignal transmitted by that communicating party.

In Embodiment 1 through Embodiment 5, the number of transmission pathestimation preamble symbols selected by a communicating party isrecognized by means of information indicating the number of transmissionpath estimation preamble symbols from that communicating party. In theabove embodiments, two kinds of transmission path estimation preamble(that is, transmission path estimation preambles whose number of symbolsis 1 and 2 respectively) are used, and therefore the amount ofinformation that needs to be provided for transmission of theabove-mentioned information is only 1 bit for one communicating party.Thus, considering the improvement in transmission efficiency achieved bythe present invention, this amount of information is very small.

However, in order to achieve a further improvement in transmissionefficiency, it is necessary to be able to recognize the number oftransmission path estimation preamble symbols selected by acommunicating party without using this kind of information. If this kindof information can be rendered unnecessary, with a total of 200communicating parties, for example, the amount of information can bereduced by 200 bits.

Thus, in this embodiment, the number of transmission path estimationpreamble symbols selected by a communicating party is estimated by meansof a correlation value calculated using a burst unit signal transmittedby that communicating party and an IFFT-processed transmission pathestimation preamble.

FIG. 14 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 6 of the presentinvention. Parts in FIG. 14 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein, in an OFDM communication apparatus according toEmbodiment 1, the memory 114 is eliminated, a synchronization section1301 is used instead of synchronization section 108, a selection section1303 is used instead of selection section 111, and a correlation peaknumber detection section 1302 is additionally provided.

The synchronization section 1301 has the same kind of configuration assynchronization section 108 in Embodiment 1, except that it outputs acalculated correlation value to the correlation peak number detectionsection 1302. The correlation peak number detection section 1302 detectsthe number of peaks in correlation values calculated by thesynchronization section 1301, and outputs the detection result toselection section 1303. Selection section 1303 has the same kind ofconfiguration as selection section 111 in Embodiment 1, except that itoutputs either a received signal from the synchronization section 1301or an averaged received signal from the averaging section 110 based onthe result of detection by the correlation peak number detection section1302.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described with additional reference to FIG.15 through FIG. 17. FIG. 15 is a schematic diagram illustratingcorrelation values calculated by the synchronization section 1301 of anOFDM communication apparatus according to Embodiment 6 of the presentinvention (first example), FIG. 16 is a schematic diagram illustratingcorrelation values calculated by the synchronization section 1301 of anOFDM communication apparatus according to Embodiment 6 of the presentinvention (second example), and FIG. 17 is a block diagram showing theconfiguration of the correlation peak number detection section 1302 inan OFDM communication apparatus according to Embodiment 6 of the presentinvention.

In the transmitting system in this embodiment, unlike Embodiment 1,information indicating the number of transmission path estimationpreamble symbols selected by selection section 102 is not sent to acommunicating party either at the start of communication or atpredetermined time intervals.

Meanwhile, in the receiving system, the number of transmission pathestimation preamble symbols selected by a communicating party can becalculated using a received signal relating to that communicating partyand an IFFT-processed transmission path estimation preamble.Specifically, when the number of transmission path estimation preamblesymbols selected by a communicating party is 2 (1), a signalcorresponding to a transmission path estimation preamble equivalent totwo symbols (one symbol) is contained in the received signal, andtherefore two peaks are (one peak is) generated in the calculatedcorrelation values.

Thus, in the correlation peak number detection section 1302, acorrelation value calculated by the synchronization section 1301 iscompared with a threshold value by a size comparison section 1601, asshown in FIG. 17. The result of the comparison is output to a counter1602. In the counter 1602, the number of times the correlation value isgreater than or equal to the threshold value is counted, based on thecomparison results. The result of the count is output to a determinationsection 1603. In the determination section 1603, the result of the countis determined, and the number of peaks in the correlation values isdetected. The detection result is output to selection section 1303. Thenumber of transmission path estimation preamble symbols is detected byselection section 1303 in accordance with this detection result. That isto say, when the number of peaks in the correlation values is 1, thenumber of transmission path estimation preamble symbols is recognized asbeing 1, and when the number of peaks in the correlation values is 2,the number of transmission path estimation preamble symbols isrecognized as being 2. Subsequent operation by selection section 1303 isthe same as in Embodiment 1, and a detailed description is omitted here.

Thus, in this embodiment, the number of transmission path estimationpreamble symbols selected by a communicating party is not recognized bymeans of notification from that communicating party, but is estimated bymeans of a correlation value calculated using a signal transmitted bythat communicating party. By this means, transmission of informationindicating the number of transmission path estimation preamble symbolsis rendered unnecessary, thereby making it possible to further prevent afall in transmission efficiency.

EMBODIMENT 7

In this embodiment, a case is described where erroneous estimation ofthe number of transmission path estimation preamble symbols isprevented.

In Embodiment 6, the number of transmission path estimation preamblesymbols selected by a communicating party is estimated by means of acorrelation value calculated using a signal transmitted by thatcommunicating party. However, when channel quality is poor, it may notbe possible to estimate the number of transmission path estimationpreamble symbols accurately due to the occurrence of error in thecalculated correlation value. As a result, there is a risk ofdeterioration of demodulated signal error rate characteristics.

Thus, in this embodiment, when channel quality is poor the number oftransmission path estimation preamble symbols inserted in a burst unitsignal is increased (here, to two symbols) on a fixed basis. By thismeans, it is possible to prevent erroneous estimation of the number oftransmission path estimation preamble symbols. Inserting a transmissionpath estimation preamble with a large number of symbols in a signal fora communicating party for which communication quality is poor in thisway will be described below taking a case where this procedure isapplied to Embodiment 1 as an example (it goes without saying that thisprocedure can also be applied to Embodiment 2 through Embodiment 6).

FIG. 18 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 7 of the presentinvention. Parts in FIG. 18 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein an OFDM communication apparatus according toEmbodiment 1 is provided with a size comparison section 1701 thatcompares quality information from a detection section 115 with athreshold value and outputs the result of the comparison to a selectionsection 1702, which outputs either a transmission path estimationpreamble from selection section 102 or a transmission path estimationpreamble whose number of symbols is 2 to the conversion section 103based on the result of a comparison by the size comparison section 1701.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 1. Quality information generated by thedetection section 115 is output to the size comparison section 1701 andselection section 102. The same kind of operation is performed as inEmbodiment 1, and a transmission path estimation preamble is output fromselection section 102 to selection section 1702.

In the size comparison section 1701, quality information is comparedwith a threshold value. The result of this comparison is output toselection section 1702. In selection section 1702, either thetransmission path estimation preamble from selection section 102 or atransmission path estimation preamble whose number of symbols is 2 isselected as the transmission path estimation preamble to be output tothe conversion section 103. Specifically, a transmission path estimationpreamble whose number of symbols is 2 is selected only when channelquality is identified as being poor according to the result of thecomparison by the size comparison section 1701. In other cases (that is,when channel quality is identified as being good), the transmission pathestimation preamble from selection section 102 is selected as thetransmission path estimation preamble to be output to the conversionsection 103. The transmission path estimation preamble selected byselection section 1702 is output to the conversion section 103.

In this embodiment, in the same way as in Embodiment 1, informationindicating the number of transmission path estimation preamble symbolsselected by selection section 1702 is transmitted to a communicatingparty via a specific channel at the start of communication and atpredetermined time intervals. Information indicating the transmissionpath estimation preamble transmitted in this way is stored in the memory114 of the transmitting system at the communicating party.

Thus, in this embodiment, the number of transmission path estimationpreamble symbols inserted in a burst unit signal is increased on a fixedbasis for a communicating party for which communication quality is poor.By this means, the number of transmission path estimation preamblescontained in a received signal can be recognized reliably by thecommunicating party, enabling deterioration of demodulated signal errorrate characteristics to be prevented. As increasing the number oftransmission path estimation preamble symbols on a fixed basis islimited to a burst unit signal for a communicating party for whichcommunication quality is poor, the fall in overall transmissionefficiency is very small.

EMBODIMENT 8

In this embodiment, a case is described where the number of transmissionpath estimation preamble symbols selected by a communicating party isnot recognized by means of notification from that communicating party,but is estimated using a signal transmitted by that communicating party,while the circuit scale is reduced.

In Embodiment 6, the number of transmission path estimation preamblesymbols selected by a communicating party is estimated by means of acorrelation value calculated using a signal transmitted by thatcommunicating party. However, when the number of FFT samples is 64, forexample, 64 complex correlators are necessary in order to configure acorrelator that calculates correlation values. Consequently, the circuitscale of an OFDM communication apparatus provided with this kind ofcorrelator is very large.

Thus, in this embodiment, when the reception level of a received signalfor a communicating party is greater than or equal to a threshold value,it is estimated that a transmission path estimation preamble with asmaller number of symbols has been inserted in a received signal forthat communicating party. This estimation is made in consideration ofthe fact that the communicating party has selected a transmission pathestimation preamble with a smaller number of symbols due to the factthat channel quality is good for the communicating party.

Conversely, when the reception level of a received signal for acommunicating party is less than a threshold value, it is estimated thata transmission path estimation preamble with a larger number of symbolshas been inserted in a received signal for that communicating party.This estimation is made in consideration of the fact that thecommunicating party has selected a transmission path estimation preamblewith a larger number of symbols due to the fact that channel quality ispoor for the communicating party.

FIG. 19 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 8 of the presentinvention. Parts in FIG. 19 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein, in an OFDM communication apparatus according toEmbodiment 1, the memory 114 is eliminated, a selection section 1803 isused instead of selection section 111, and a level calculation section1801 and size comparison section 1802 are additionally provided.

Selection section 1803 has the same kind of configuration as selectionsection 111 in Embodiment 1, except that it outputs either a receivedsignal from a synchronization section 108 or an averaged received signalfrom an averaging section 110 based on the result of a comparison by thesize comparison section 1802. The level calculation section 1801calculates the reception level of a received signal from thesynchronization section 108, and outputs the calculated reception levelto the size comparison section 1802. The size comparison section 1802compares the calculated reception level with a threshold value, andoutputs the result of the comparison to selection section 1803.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 1. In the transmitting system in thisembodiment, unlike Embodiment 1, information indicating the number oftransmission path estimation preamble symbols selected by selectionsection 102 is not sent to a communicating party either at the start ofcommunication or at predetermined time intervals.

Meanwhile, in the receiving system, the reception level of a receivedsignal from the synchronization section 108 is calculated by the levelcalculation section 1801. The calculated reception level is stored inthis level calculation section 1801 and is also output to the sizecomparison section 1802. This level calculation section 1801 cancomprise a single memory that stores the calculated reception level(I²+Q²). Alternatively, this level calculation section 1801 can comprisetwo multipliers (that is, a multiplier that multiplies the receivedsignal I component by itself, and a multiplier that multiplies thereceived signal Q component by itself) and one adder (that is, an adderthat adds together the results from the two multipliers).

In the size comparison section 1802, the calculated reception level iscompared with a threshold value. The result of the comparison is outputto selection section 1803. In selection section 1803, the number oftransmission path estimation preamble symbols is detected based on theresult of the comparison by the size comparison section 1802.Specifically, when the received signal reception level is higher than orequal to the threshold value, it is detected that a transmission pathestimation preamble with a smaller number of symbols (here, one) hasbeen inserted in the received signal. Conversely, when the receivedsignal reception level is lower the threshold value, it is detected thata transmission path estimation preamble with a larger number of symbols(here, two) has been inserted in the received signal. Subsequentoperation by selection section 1803 is the same as in Embodiment 1, anda detailed description is omitted here.

Thus, in this embodiment, the number of transmission path estimationpreamble symbols selected by a communicating party is not recognized bymeans of notification from that communicating party, but is estimated bymeans of the reception level of a signal transmitted by thatcommunicating party. By this means, transmission of informationindicating the number of transmission path estimation preamble symbolsis rendered unnecessary, thereby making it possible to further prevent afall in transmission efficiency. Moreover, estimating the number oftransmission path estimation preambles selected by the above-mentionedcommunicating party without using a correlator enables the circuit scaleto be reduced compared with Embodiment 6.

EMBODIMENT 9

In this embodiment, a case is described where the number of transmissionpath estimation preamble symbols selected by a communicating party isnot recognized by means of notification from that communicating party,but is estimated using a signal transmitted by that communicating party,while processing delay is reduced.

In Embodiment 6, the number of transmission path estimation preamblesymbols selected by a communicating party is estimated using acorrelation value between a received signal for that communicating partyand an IFFT-processed transmission path estimation preamble. In thiscase, a processing delay time equivalent to one OFDM signal occurs.

In the above embodiment, when a transmission path estimation preamblewhose number of symbols is 2 is used, the kind of format shown in FIG. 5is used. Recently, the kind of format shown in FIG. 20 has been proposedas a format for use when a transmission path estimation preamble whosenumber of symbols is 2 is used. FIG. 20 is a schematic diagramillustrating a burst unit signal format in an OFDM communicationapparatus according to Embodiment 9 of the present invention.

As shown in FIG. 20, the first transmission path estimation preamblesymbol serves as a guard interval for the second transmission pathestimation preamble symbol. Thus, it is not necessary to insert a guardinterval between the first transmission path estimation preamble symboland the second transmission path estimation preamble symbol. Therefore,in the format shown in FIG. 20, unlike the format shown in FIG. 5, twoguard intervals are inserted together immediately before the firsttransmission path estimation preamble symbol. Thus, in this embodiment,a case is described where the kind of format shown in FIG. 20 is usedwhen using a transmission path estimation preamble whose number ofsymbols is 2.

When a communicating party performs transmission in accordance with thekind of format shown in FIG. 20 (that is, when a transmission pathestimation preamble whose number of symbols is 2 is inserted in a burstunit signal), a peak of a predetermined size occurs in correlationvalues between the received signal and guard intervals at a timingcorresponding to the center of the guard intervals, as shown in FIG. 20.On the other hand, when a communicating party performs transmission inaccordance with the kind of format shown in FIG. 6 (that is, when atransmission path estimation preamble whose number of symbols is 1 isinserted in a burst unit signal), the above-mentioned peak of apredetermined size does not occur in correlation values between thereceived signal and guard intervals. In this embodiment, the number oftransmission path estimation preamble symbols selected by acommunicating party is estimated using a correlation value between areceived signal and guard interval.

FIG. 21 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 9 of the presentinvention. Parts in FIG. 21 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein, in an OFDM communication apparatus according toEmbodiment 1, the memory 114 is eliminated, a selection section 2001 isused instead of selection section 102, a selection section 2006 is usedinstead of selection section 111, a GI insertion section 2002 and GIinsertion section 2007 are used instead of GI insertion section 105, anda selection section 2003, correlation section 2004, and size comparisonsection 2005 are additionally provided.

Selection section 2001 has the same kind of configuration as selectionsection 102 in Embodiment 1 (FIG. 3), except that it notifies thetransmission path estimation preamble selection result to selectionsection 2003.

GI insertion section 2002 inserts guard intervals in a generated OFDMsignal in accordance with the kind of format shown in FIG. 20, andgenerates a transmit signal. GI insertion section 2007 inserts guardintervals in a generated OFDM signal in accordance with the kind offormat shown in FIG. 6, and generates a transmit signal.

Selection section 2003 selects an OFDM signal with guard intervalsinserted by GI insertion section 2007 or GI insertion section 2002 asthe transmit signal, based on the result of transmission path estimationpreamble selection by selection section 2001.

The correlation section 2004 outputs a correlation value between thereceived signal from a synchronization section 108 and guard intervals.The size comparison section 2005 compares a peak in correlation valuescalculated by the correlation section 2004 with a threshold value.Selection section 2006 has the same kind of configuration as selectionsection 111 in Embodiment 1, except that it outputs either a receivedsignal from the synchronization section 108 or an averaged receivedsignal from an averaging section 110 based on the result of a comparisonby the size comparison section 2005.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 1. In the transmitting system in thisembodiment, unlike Embodiment 1, information indicating the number oftransmission path estimation preamble symbols selected by selectionsection 2001 is not sent to a communicating party either at the start ofcommunication or at predetermined time intervals. Also, the result oftransmission path estimation preamble selection by selection section2001 is output to selection section 2003.

In GI insertion section 2002, a transmit signal is generated byinserting guard intervals in an OFDM signal generated by an IFFT section104 in accordance with the kind of format shown in FIG. 20. In GIinsertion section 2007, a transmit signal is generated by insertingguard intervals in an OFDM signal generated by the IFFT section 104 inaccordance with the kind of format shown in FIG. 6. The guard intervalinserted immediately before a transmission path estimation preamble inthe format shown in FIG. 20 is generated by providing two consecutiveguard intervals inserted immediately before a transmission pathestimation preamble in the format shown in FIG. 6.

In selection section 2003, the transmit signal generated by either GIinsertion section 2002 or GI insertion section 2007 is selected as thetransmit signal to be actually transmitted based on the result oftransmission path estimation preamble selection by selection section2001. Specifically, when a transmission path estimation preamble whosenumber of symbols is 2 (1) is selected by selection section 2001, thetransmit signal generated by GI insertion section 2002 (GI insertionsection 2007) is selected as the transmit signal to be transmitted. Thetransmit signal selected by selection section 2003 undergoespredetermined transmission processing, and is then transmitted to thecommunicating party via antenna 106.

Meanwhile, in the receiving system, a correlation value between areceived signal from the synchronization section 108 and a guardinterval is calculated in the correlation section 2004. It goes withoutsaying that a guard interval here has the same signal pattern as a guardinterval used by GI insertion section 2007 or S/P conversion section2002 in the transmitting system.

In the size comparison section 2005, a peak in the correlation valuescalculated by the correlation section 2004 is compared with a thresholdvalue. The result of the comparison is output to selection section 2006.In selection section 2006, the number of transmission path estimationpreamble symbols is detected based on the result of the comparison bythe size comparison section 2005. That is to say, when a peak ofpredetermined size occurs in the correlation values, it is recognizedthat the number of transmission path estimation preamble symbols is 2,and when a peak of predetermined size does not occur in the correlationvalues, it is recognized that the number of transmission path estimationpreamble symbols is 1. Subsequent operation by selection section 2006 isthe same as in Embodiment 1, and a detailed description is omitted here.

Thus, in this embodiment, in the transmitting system the number of guardinterval symbols (in this embodiment, 1 or 2) inserted in apredetermined position in a burst unit signal (in this embodiment, inthe leading part of the burst unit signal) is varied in accordance withthe number of transmission path estimation preamble symbols selectedaccording to channel quality. Also, in the receiving system, the numberof transmission path estimation preamble symbols selected by acommunicating party is estimated in accordance with the number of peaksof predetermined size in correlation values between a received signaland guard intervals. By this means, transmission of informationindicating the number of transmission path estimation preamble symbolsis rendered unnecessary, thereby making it possible to further prevent afall in transmission efficiency.

Moreover, as a correlation value with a received signal is calculatedusing guard intervals instead of transmission path estimation preambles,and the number of transmission path estimation preamble symbols selectedby a communicating party is estimated using the calculated correlationvalue, the processing delay time incurred can be reduced to a greaterextent than in Embodiment 6.

Furthermore, the total number of multipliers, adders, and delayersrequired to configure the correlation section 2004 (number of guardinterval samples/2) can be made smaller than the total number ofmultipliers, adders, and delayers required to configure asynchronization section 108.

EMBODIMENT 10

In this embodiment, a case is described where the number of transmissionpath estimation preamble symbols selected by a communicating party isnot recognized by means of notification from that communicating party,but is estimated using a signal transmitted by that communicating party,while the apparatus scale is reduced.

In Embodiment 9, the number of guard interval symbols inserted in aburst unit signal is varied in accordance with the selected number oftransmission path estimation preamble symbols. Therefore, in Embodiment9, there is a possibility of processing in the transmitting systembecoming complex.

Thus, in this embodiment, the transmission path estimation preamblesignal pattern inserted in a burst unit signal is varied in accordancewith the selected number of transmission path estimation preamblesymbols.

FIG. 22 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 10 of the presentinvention. Parts in FIG. 22 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

An OFDM communication apparatus according to this embodiment has aconfiguration wherein, in an OFDM communication apparatus according toEmbodiment 1, the memory 114 is eliminated, a selection section 2101 isused instead of selection section 102, a synchronization section 2102 isused instead of synchronization section 108, a selection section 2105 isused instead of selection section 111, and a correlation section 2103and size comparison section 2104 are additionally provided.

Selection section 2101 has the same kind of configuration as selectionsection 102 in Embodiment 1, except that it outputs either transmissionpath estimation preamble 1 or transmission path estimation preamble 3 toa conversion section 103 based on quality information from a detectionsection 115. Here, transmission path estimation preamble 3 has a signallength twice that of transmission path estimation preamble 1 (that is,the same signal length as above-mentioned transmission path estimationpreamble 2), and has a different signal pattern from that oftransmission path estimation preamble 1. Specifically, transmission pathestimation preamble 3 is generated by combining two signals that havethe same signal length as transmission path estimation preamble 1 andhave different signal patterns. Here, the number of symbols oftransmission path estimation preamble 1 is 1, and the number of symbolsof transmission path estimation preamble 3 is 2.

The synchronization section 2102 has the same kind of configuration assynchronization section 108 in Embodiment 1, except that it does notonly calculate a correlation value between a received signal andIFFT-processed transmission path estimation preamble 1, but alsocalculates a correlation value between the received signal andIFFT-processed transmission path estimation preamble 3, and detects thetiming at which each correlation value is at a maximum.

The correlation section 2103 calculates a correlation value between thereceived signal from synchronization section 2102 and transmission pathestimation preamble 3. The size comparison section 2104 compares a peakof predetermined size in correlation values calculated by thecorrelation section 2103 with a threshold value. Selection section 2105has the same kind of configuration as selection section 111 inEmbodiment 1, except that it outputs either a received signal from thesynchronization section 2102 or an averaged received signal from theaveraging section 110 based on the result of a comparison by the sizecomparison section 2104.

Next, the operation of an OFDM communication apparatus that has theabove configuration will be described, considering only points ofdifference from Embodiment 1. In the transmitting system, when thequality of a demodulated signal in the receiving system is greater thanor equal to a threshold value, transmission path estimation preamble 1is selected. Conversely, when the quality of a demodulated signal in thereceiving system is less than the threshold value, transmission pathestimation preamble 3 is selected. The selected transmission pathestimation preamble is output to the conversion section 103.

Meanwhile, in the receiving system, a correlation value between thereceived signal and transmission path estimation preamble 1 and acorrelation value between the received signal and transmission pathestimation preamble 3 are calculated by the synchronization section2102. Also, the timing at which each calculated correlation value is ata maximum is detected. By this means, the timing at which a correlationvalue is at a maximum is detected when either transmission pathestimation preamble 1 or transmission path estimation preamble 3 isinserted in a received signal. The detected timings are output to atiming generation section 109.

In the correlation section 2103, a correlation value between thereceived signal from the correlation section 2103 and the transmissionpath estimation preamble is calculated. Here, when transmission pathestimation preamble 3 is used by the communicating party, a peak of apredetermined size occurs in calculated correlation values. Conversely,when transmission path estimation preamble 1 is used by thecommunicating party, a peak of a predetermined size does not occur incalculated correlation values. The calculated correlation value isoutput to the size comparison section 2104.

In the size comparison section 2104, a peak in correlation valuescalculated by the correlation section 2103 is compared with a thresholdvalue. The result of the comparison is output to selection section 2105.

In selection section 2105, the number of transmission path estimationpreamble symbols (that is, the transmission path estimation preambleused by the communicating party) is detected based on the result of thecomparison by the size comparison section 2104. That is to say, when apeak of a predetermined size occurs in the correlation values, it isrecognized that the number of transmission path estimation preamblesymbols is 2 (that is, that transmission path estimation preamble 3 hasbeen used by the communicating party), and when a peak of apredetermined size does not occur in the correlation values, it isrecognized that the number of transmission path estimation preamblesymbols is 1 (that is, that transmission path estimation preamble 1 hasbeen used by the communicating party). Subsequent operation by selectionsection 2105 is the same as in Embodiment 1, and a detailed descriptionis omitted here.

Thus, in this embodiment, in the transmitting system the transmissionpath estimation preamble signal pattern inserted in a burst unit signalis varied in accordance with the number of transmission path estimationpreamble symbols selected according to channel quality. Also, in thereceiving system, a correlation value between a transmission pathestimation preamble signal pattern used in the transmitting system (inthis embodiment transmission path estimation preamble 3 is used, but itis also possible for transmission path estimation preamble 1 to be used)and a received signal is calculated, and the transmission pathestimation preamble selected by the communicating party is estimatedbased on the number of peaks of predetermined size in the calculatedcorrelation values.

By this means, transmission of information indicating the number oftransmission path estimation preamble symbols is rendered unnecessary,thereby making it possible to further prevent a fall in transmissionefficiency. Moreover, the guard interval insertion method in thetransmitting system is unrelated to the selected number of transmissionpath estimation preamble symbols, and therefore the apparatusconfiguration can be simplified compared with Embodiment 9.

EMBODIMENT 11

In this embodiment, a case is described, with reference to FIG. 23 andFIG. 24, where the correlator in Embodiment 1 through Embodiment 10 isconfigured without using multipliers, thus reducing the circuit scale.FIG. 23 is a block diagram showing a sample configuration of acorrelator used in an OFDM communication apparatus, and FIG. 24 is ablock diagram showing the configuration of the correlator used in anOFDM communication apparatus according to Embodiment 11 of the presentinvention. In FIG. 23 and FIG. 24, the input signal corresponds to areceived signal from an antenna 107, and coefficient 1 throughcoefficient k correspond to an IFFT-processed transmission pathestimation preamble.

As shown in FIG. 23, multipliers are predominant in the configuration ofa correlator used in Embodiment 1 through Embodiment 10 above.Configuring a correlator that does not use multipliers is effective inreducing the circuit scale of the correlator.

An IFFT-processed transmission path estimation preamble is amulticarrier signal, and therefore its polarity varies abruptlyaccording to the sample point. Thus, signals obtained by means of a harddecision of an IFFT-processed transmission path estimation preamble canbe used as coefficient 1 through coefficient k.

In FIG. 24, determination section 2303-n (n=1 to k) performs a harddecision on coefficient n in an IFFT-processed transmission pathestimation preamble, and outputs the hard decision result to selectionsection 2302−n.

A received signal delayed by delay section 2201-(n−1) and a signalproduced by polarity inversion of this delayed received signal bypolarity inversion section 2301-n (hereinafter referred to as“inverted-polarity signal”) are input to selection section 2302-n.However, a received signal that has not been delayed is input to delaysection 2201-1. Each delay section (in FIG. 24, delay section 2201-1 anddelay section 2201-2) delays the received signal by a predeterminedtime.

When the hard decision result from determination section 2303-n is “1”,selection section 2302-n outputs the received signal delayed by delaysection 2201-(n−1) to addition section 2203-(n−1). Conversely, when thehard decision result from determination section 2303-n is “0”, selectionsection 2302-n outputs the inverted-polarity signal from polarityinversion section 2301-n to addition section 2203-(n−1).

Addition section 2203-n adds the signal selected by selection section2302-n and the signal selected by selection section 2302-(n+1). Also,addition section 2203-n (n≠k) outputs the signal resulting from theaddition to addition section 2203-(n+1). However, addition section2203-(k−1) outputs the signal resulting from the addition as thecorrelation result.

Thus, according to this embodiment, a correlator is configured withoutusing multipliers, thus enabling the apparatus scale to be reducedcompared with Embodiment 1 through Embodiment 10.

EMBODIMENT 12

In this embodiment, a case is described where OFDM-CDMA communication isapplied to Embodiment 1 through Embodiment 10. OFDM-CDMA communicationcan be applied to any of Embodiments 1 through 10. In this embodiment, acase where OFDM-CDMA communication is applied to Embodiment 1 will bedescribed as an example, with reference to FIG. 25.

FIG. 25 is a block diagram showing the configuration of an OFDMcommunication apparatus according to Embodiment 12 of the presentinvention. Parts in FIG. 25 identical to those in Embodiment 1 (FIG. 3)are assigned the same codes as in FIG. 3 and their detailed explanationsare omitted.

In the transmitting system, a serial/parallel conversion section(hereinafter referred to as “S/P section”) 2401 converts an informationsignal that has undergone modulation processing from a single-sequencesignal to a plurality of sequences of signals. For the sake ofsimplicity, the number of sequences will be designated n. Sequence 1through sequence n signals are output to sequence-specific spreadingsections 2402. Spreading sections 2402 perform spreading processing onsequence 1 through sequence n signals using spreading code 1 throughspreading code n respectively. An addition section 2403 multiplexessequence 1 through sequence n signals that have undergone spreadingprocessing. By this means a code-multiplexed signal (hereinafterreferred to as “multiplex signal”) is obtained. The multiplex signal isoutput to a conversion section 103.

An S/P section 2404 decomposes the signal from the conversion section103 (a multiplex signal from the addition section 2403 or a transmissionpath estimation preamble from a selection section 102) for each despreadsignal (that is, on a chip-by-chip basis) and generates a plurality ofsequences of signals equivalent to the spreading ratio (k). That is tosay, S/P section 2404 generates a first chip through k'th chip of thesignal from the conversion section 103 as a plurality of sequences ofsignals.

An IFFT section 104 performs frequency division multiplexing processingby executing IFFT processing on the first chip through k'th chip of thesignal from the conversion section 103. That is to say, the IFFT section104 generates an OFDM signal in which the above-mentioned first chipthrough k'th chip are superimposed on subcarrier 1 through subcarrier krespectively. The generated OFDM signal is output to a GI insertionsection 105.

Meanwhile, in the receiving system, an FFT section 112 extracts signalstransmitted by subcarrier 1 through subcarrier k by performing FFTprocessing on a received signal from a selection section 111 based on atiming signal generated by a timing generation section 109. Transmissionpath compensation sections 2405 perform transmission path compensationprocessing on signals transmitted by subcarrier 1 through subcarrier k.Details of this transmission path compensation processing will be givenlater herein.

A parallel/serial conversion section (hereinafter referred to as “P/Ssection”) 2406 converts a plurality of sequences of signals that haveundergone transmission path compensation (that is, signals transmittedby subcarrier 1 through subcarrier k) to a single-sequence signal, andoutputs this to despreading sections 2407. That is to say, P/S section2406 outputs the first chip in the signal from each transmission pathcompensation section 2405 in time t1, and outputs the k'th chip in thatsignal in time tk.

Despreading sections 2407 generate demodulated signal 1 throughdemodulated signal n by performing despreading processing on thesingle-sequence signal from P/S section 2406 using despreading code 1through spreading code n respectively. A P/S section 2408 convertsdemodulated signal 1 through demodulated signal n (that is, a pluralityof sequences of signals) from despreading sections 2407 to asingle-sequence signal, and output this to a demodulation section 113 asa demodulated signal.

Next, details of the transmission path compensation section 2405 will begiven with reference to FIG. 26 through FIG. 28. FIG. 26 is a schematicdiagram illustrating reception levels of signals transmitted by eachsubcarrier in an OFDM communication apparatus according to Embodiment 12of the present invention (first example), FIG. 27 is a schematic diagramillustrating reception levels of signals transmitted by each subcarrierin an OFDM communication apparatus according to Embodiment 12 of thepresent invention (second example), and FIG. 28 is a block diagramshowing the configuration of the transmission path compensation section2405 in an OFDM communication apparatus according to Embodiment 12 ofthe present invention.

In a normal OFDM system, when transmission path compensation isperformed, phase and amplitude compensation is carried out for signalstransmitted by each subcarrier. However, in an OFDM-CDMA system, eachchip (here, the first chip through k'th chip) of acode-division-multiplexed signal (here, multiplex signal) issuperimposed on a different subcarrier. Therefore, if phase andamplitude compensation is carried out for signals transmitted by eachsubcarrier, a frequency diversity effect is no longer obtained.

On the other hand, when only phase compensation is carried out forsignals transmitted by each subcarrier, orthogonality is lost betweenspreading codes. It therefore becomes necessary to decrease the level ofcode multiplexing (that is, the total number of signals multiplexed bythe addition section 2403 in FIG. 25), and consequently spectralefficiency falls. As explained above, in an OFDM-CDMA system it isdifficult to perform the transmission path compensation used in a normalOFDM system.

Thus, in this embodiment, it is determined whether phase compensationonly, or phase and amplitude compensation, is to be performed on signalstransmitted by each subcarrier, in accordance with the reception level(for example, the average reception level) of signals transmitted byeach subcarrier. Specifically, when the above-mentioned reception levelis greater than or equal to a threshold value (FIG. 27), phase andamplitude compensation is performed on signals transmitted by eachsubcarrier, and, conversely, when that reception level is less than thethreshold value (FIG. 27), only phase compensation is performed onsignals transmitted by each subcarrier. By this means, it is possible toachieve both an improvement in frequency diversity effect and animprovement in spectral efficiency. In a normal OFDM system, a pluralityof signals cannot be multiplexed in a signal transmitted by onesubcarrier, and therefore the above-described effects cannot beobtained.

A transmission path compensation circuit according to this embodimentwill now be described with reference to FIG. 28. Here, of the ktransmission path compensation circuits provided, the transmission pathcompensation circuit that performs transmission path compensation forthe signal transmitted by subcarrier k will be considered as an example.A post-FFT signal—that is, a signal transmitted by subcarrier k—isoutput to a power calculation section 2701, amplitude calculationsection 2702, multiplication section 2706, and multiplication section2707. The power calculation section 2701 calculates the power of thesignal transmitted by subcarrier k, and outputs the calculated power toa size comparison section 2703 and selection section 2704. The amplitudecalculation section 2702 calculates the amplitude of the signaltransmitted by subcarrier k, and outputs the calculated amplitude to theselection section 2704.

The size comparison section 2703 compares the power (for example,average power) calculated by the power calculation section 2701 with athreshold value, and outputs the result of the comparison to theselection section 2704. Based on the result of the comparison by thesize comparison section 2703, the selection section 2704 outputs eitherthe power calculated by the power calculation section 2701 or theamplitude calculated by the amplitude calculation section 2702 to adivision section 2705. Specifically, the selection section 2704 outputsthe power calculated by the power calculation section 2701 to thedivision section 2705 when that power is greater than or equal to athreshold value, and conversely, outputs the amplitude calculated by theamplitude calculation section 2702 to the division section 2705 when theabove-mentioned power is less than the threshold value.

Multiplication section 2706 performs multiplication of the signaltransmitted by subcarrier k and the transmission path estimationpreamble, by which means a transmission path estimation result isobtained. The obtained transmission path estimation result is output tothe division section 2705.

The division section 2705 performs division on the transmission pathestimation result from multiplication section 2706 using the calculatedamplitude or power from the selection section 2704. The result of thedivision is output to multiplication section 2707. Multiplicationsection 2707 multiplies the signal transmitted by subcarrier k by theresult of the division by the division section 2705. By this means asignal that has undergone transmission path compensation is obtained.

As explained above, it is possible to apply OFDM-CDMA communication toEmbodiment 1 through Embodiment 10. In this case, it is possible toachieve both an improvement in frequency diversity effect and animprovement in spectral efficiency by performing only phasecompensation, or phase and amplitude compensation, for a signaltransmitted by a predetermined subcarrier, in accordance with thereception level of the signal transmitted by that predeterminedsubcarrier.

An OFDM communication apparatus according to the above embodiments canbe installed in a communication terminal apparatus and base stationapparatus in a digital mobile communication system. By this means, thatcommunication terminal apparatus and base station apparatus can achieveboth an improvement in demodulated signal error rate characteristics andan improvement in information signal transmission efficiency, thusenabling good communication to be performed efficiently.

In the above embodiments, a case has been described where the presentinvention is applied to an OFDM communication apparatus that executesinverse fast Fourier transform (IFFT) processing on the transmittingside and also executes fast Fourier transform (FFT) processing on thereceiving side, but the present invention can also be applied to an OFDMcommunication apparatus that executes inverse discrete Fourier transform(IDFT) processing on the transmitting side and also executes discreteFourier transform (DFT) processing on the receiving side. The key pointis that the present invention can be widely applied to OFDMcommunication apparatuses that perform inverse Fourier transformprocessing on the transmitting side and perform Fourier transformprocessing on the receiving side.

The present invention is not limited to the above-described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

An OFDM communication apparatus of the present invention has aconfiguration comprising determining section that determine a number ofknown signals for transmission path estimation to be inserted in atransmit signal based on channel quality with respect to a communicatingparty, and generating section that perform inverse Fourier transformprocessing on an information signal and the number of known signals fortransmission path estimation determined by the determining section andgenerating a transmit signal for the communicating party.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section estimates a deteriorationfactor for a demodulated signal at a communicating party using channelquality with respect to the communicating party, and determines a numberof known signals for transmission path estimation based on the estimateddeterioration factor.

According to these configurations, it is possible to achieve both animprovement in demodulated signal error rate characteristics and animprovement in information signal transmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section detects channel quality withrespect to a communicating party using a reception level in the OFDMcommunication apparatus itself or a reception level at the communicatingparty.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section detects channel quality withrespect to a communicating party using received signal delay variance inthe OFDM communication apparatus itself or received signal delayvariance at the communicating party.

An OFDM communication apparatus of the present invention has aconfiguration further comprising delay variance detecting section thatdetect received signal delay variance, wherein determining sectiondetects channel quality with respect to a communicating party using theresult of normalizing received signal delay variance in the OFDMcommunication apparatus itself according to the level of the receivedsignal, or the result of normalizing received signal delay variance at acommunicating party according to the level of the received signal at thecommunicating party.

According to these configurations, channel quality with respect to acommunicating party can be detected reliably, and it is thereforepossible to achieve both an improvement in demodulated signal error ratecharacteristics and an improvement in information signal transmissionefficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section determines a number of knownsignals for transmission path estimation based on the total number ofcommunicating parties.

According to this configuration, by increasing the number of knownsignals for transmission path estimation inserted in a transmit signalfor a predetermined communicating party when the total number ofcommunicating parties actually performing communication is small, it ispossible to reduce time slots not used for communication in each frame,and it is also possible to improve demodulated signal error ratecharacteristics for the predetermined communicating party. Conversely,by decreasing the number of known signals for transmission pathestimation inserted in a transmit signal for a predeterminedcommunicating party when the total number of communicating partiesactually performing communication is large, it is possible to increasethe total number transmit signals contained in each frame, and soimprove transmission efficiency. It is therefore possible to achieveboth an improvement in demodulated signal error rate characteristics andan improvement in information signal transmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section determines a number of knownsignals for transmission path estimation to be inserted in a transmitsignal based on a communication channel used for transmission of thetransmit signal to a communicating party.

According to this configuration, it is possible to improve thedemodulated signal error rate in a specific channel for which bettercommunication quality is required with almost no effect on overalltransmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein determining section increases the number of knownsignals for transmission path estimation on a fixed basis when channelquality with respect to a communicating party is poor.

According to this configuration, it is possible to identify reliably thenumber of known signals for transmission path estimation contained in areceived signal, and so to prevent deterioration of demodulated signalerror rate characteristics.

An OFDM communication apparatus of the present invention has aconfiguration comprising receiving section that receive a transmitsignal obtained by performing inverse Fourier transform processing of asignal containing a number of known signals for transmission pathestimation determined in accordance with channel quality with respect toa communicating party, detecting section that detect the number of knownsignals for transmission path estimation inserted in the transmit signalbased on correlation values between a received signal and known signalsfor transmission path estimation that have undergone inverse Fouriertransform processing, and demodulating section that perform transmissionpath compensation on the received signal based on the number of knownsignals for transmission path estimation detected by the detectingsection and demodulating the received signal.

According to this configuration, the number of known signals fortransmission path estimation selected by a communicating party is notidentified by section of notification from the communicating party, butis estimated by section of a correlation value calculated using a signaltransmitted by the communicating party. By this section, transmission ofinformation indicating the number of known signals for transmission pathestimation is rendered unnecessary, thereby making it possible tofurther prevent a fall in transmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration comprising receiving section that receive a transmitsignal obtained by performing inverse Fourier transform processing of asignal containing a number of known signals for transmission pathestimation determined in accordance with channel quality with respect toa communicating party, detecting section that detect the number of knownsignals for transmission path estimation inserted in the transmit signalbased on the received signal reception level, and demodulating sectionthat perform transmission path compensation on the received signal basedon the number of known signals for transmission path estimation detectedby the detecting section and demodulating the received signal.

According to this configuration, the number of known signals fortransmission path estimation selected by a communicating party is notidentified by section of notification from the communicating party, butis estimated based on the reception level of a signal transmitted by thecommunicating party. By this means, transmission of informationindicating the number of known signals for transmission path estimationis rendered unnecessary, thereby making it possible to further prevent afall in transmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein generating section inserts guard intervals basedon a number determined by determining section in predetermined positionsin a transmit signal.

An OFDM communication apparatus of the present invention has aconfiguration comprising detecting section that detect, based oncorrelation values between a received signal for a transmit signal ofthe OFDM communication apparatus and guard intervals, the number ofknown signals for transmission path estimation inserted in the transmitsignal, and demodulating section that perform transmission pathcompensation on the received signal based on the number of known signalsfor transmission path estimation detected by the detecting section anddemodulating the received signal.

According to these configurations, in the transmitting system, thenumber of guard intervals inserted in predetermined positions in atransmit signal is varied in accordance with the number of known signalsfor transmission path estimation selected according to channel quality.Also, in the receiving system, the number of known signals fortransmission path estimation selected by a communicating party isestimated in accordance with the number of peaks of predetermined sizein correlation values between a received signal and guard intervals. Bythis means, transmission of information indicating the number of knownsignals for transmission path estimation is rendered unnecessary,thereby making it possible to further prevent a fall in transmissionefficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein generating section varies the signal pattern of aknown signal for transmission path estimation based on a numberdetermined by determining section.

An OFDM communication apparatus of the present invention has aconfiguration comprising detecting section that detect, based oncorrelation values between a received signal for a transmit signal ofthe OFDM communication apparatus and signals resulting from inverseFourier transform processing of known signals for transmission pathestimation used by generating section, the number of known signals fortransmission path estimation inserted in the transmit signal, anddemodulating section that perform transmission path compensation on thereceived signal based on the number of known signals for transmissionpath estimation detected by the detecting section and demodulating thereceived signal.

According to these configurations, in the transmitting system, thesignal pattern of a known signal for transmission path estimationinserted in a transmit signal is varied in accordance with the number ofknown signals for transmission path estimation selected based on channelquality. Also, in the receiving system, a correlation value between thesignal pattern of a known signal for transmission path estimation usedin the transmitting system and a received signal is calculated, and theknown signal for transmission path estimation selected by acommunicating party is estimated in accordance with the number of peaksof predetermined size in calculated correlation values. By this means,transmission of information indicating the number of known signals fortransmission path estimation is rendered unnecessary, thereby making itpossible to further prevent a fall in transmission efficiency.

An OFDM communication apparatus of the present invention has aconfiguration wherein generating section comprises converting sectionthat convert an information signal from a single-sequence signal to aplurality of sequences of signals, spreading section that performspreading processing on the signal of each sequence using mutuallydifferent spreading codes, and multiplexing section that multipliessequences of signals that have undergone spreading processing andgenerating a multiplex signal; and inverse Fourier transform processingis performed using the generated multiplex signal and the number ofknown signals for transmission path estimation determined by determiningsection.

According to this configuration, the above-described OFDM communicationapparatus can also be applied to OFDM-CDMA communications.

A communication terminal apparatus of the present invention has aconfiguration provided with an above-described OFDM communicationapparatus. A base station apparatus of the present invention has aconfiguration provided with an above-described OFDM communicationapparatus.

According to these configurations, by incorporating an OFDMcommunication apparatus that achieves both an improvement in demodulatedsignal error rate characteristics and an improvement in informationsignal transmission efficiency, it is possible to provide acommunication terminal apparatus and base station apparatus that enablegood communication to be performed efficiently.

An OFDM communication method of the present invention comprises adetermining step of determining a number of known signals fortransmission path estimation to be inserted in a transmit signal basedon channel quality with respect to a communicating party, and agenerating step of performing inverse Fourier transform processing on aninformation signal and the number of known signals for transmission pathestimation determined in the determining step and generating a transmitsignal for the communicating party.

In an OFDM communication method of the present invention, a determiningstep estimates a deterioration factor for a demodulated signal at acommunicating party using channel quality with respect to thecommunicating party, and determines a number of known signals fortransmission path estimation based on the estimated deteriorationfactor.

According to these methods, it is possible to achieve both animprovement in demodulated signal error rate characteristics and animprovement in information signal transmission efficiency.

As described above, an OFDM communication apparatus of the presentinvention can achieve both an improvement in demodulated signal errorrate characteristics and an improvement in information signaltransmission efficiency.

This application is based on Japanese Patent Application No. 2000-351766filed on Nov. 17, 2000, entire contents of which are expresslyincorporated by reference herein.

INDUSTRIAL APPLICABILITY

The present invention is applicable to cases where coherent detection isperformed by an OFDM communication apparatus.

1. An orthogonal frequency division multiplexing transmission apparatusfor transmitting an orthogonal frequency division multiplexing signalincluding in all subcarriers known signals for transmission pathestimation used for transmission path estimation by a communicatingparty, the orthogonal frequency division multiplexing transmissionapparatus comprising: an information signal modulation section thatmodulates an information signal including user data transmitted in auser channel and an information signal formed with control data alonetransmitted in a specific channel where higher communication qualitythan the user channel is required; a determining section that determinesthe number of the known signals for transmission path estimation to beinserted in the information signal transmitted in the user channel anddetermines that the number of the known signals for transmission pathestimation to be inserted in the information signal transmitted in thespecific channel is greater than the number of the known signals fortransmission path estimation to be inserted in the information signaltransmitted in the user channel; a transmission path estimation knownsignal inserting section that inserts the determined numbers of theknown signals for transmission path estimation in the information signalof the user channel and the information signal of the specific channel;and a transmit signal generating section that generates an orthogonalfrequency division multiplexing transmit signal by performing inverseFourier transform processing on the information signals and the knownsignals for transmission path estimation outputted from the transmissionpath estimation known signal inserting section.
 2. The orthogonalfrequency division multiplexing transmission apparatus according toclaim 1, wherein the determining section adaptively changes the numberof the known signals for transmission path estimation to be inserted inthe information signal transmitted in the user channel based on receivedquality.
 3. The orthogonal frequency division multiplexing transmissionapparatus according to claim 1, wherein the specific channel comprisesbroadcast channels, control channels or retransmission channels.
 4. Anorthogonal frequency division multiplexing transmission method oftransmitting an orthogonal frequency division multiplexing signalincluding in all subcarriers known signals for transmission pathestimation used for transmission path estimation by a communicatingparty, the orthogonal frequency division multiplexing transmissionmethod comprising the steps of: modulating an information signalincluding user data transmitted in a user channel and an informationsignal formed with control data alone transmitted in a specific channelwhere higher communication quality than the user first channel isrequired; determining the number of the known signals for transmissionpath estimation to be inserted in the information signal transmitted inthe user channel and determining that the number of the known signalsfor transmission path estimation to be inserted in the informationsignal transmitted in the specific channel is greater than the number ofthe known signals for transmission path estimation to be inserted in theinformation signal transmitted in the user channel; and inserting thedetermined numbers of the known signals for transmission path estimationin the information signal of the user channel and the information signalof the specific channel, and generating an orthogonal frequency divisionmultiplexing transmit signal by performing inverse Fourier transformprocessing on the information signal of the user channel and theinformation signal of the specific channel.